How sound travels?

[Ken]

There's times when the air is cold that I can hear the train horns miles out, almost to the point I can guess check crossing they are at. Other times, I can hear a low hum of the locomotives, but cannot hear the horns. My imagination?

Probably not your imagination. The sounds of the locomotives are probably much lower frequency (lower pitch) than that of the horns. Lower frequency sound waves to travel further and also interact less (reflect, refract, etc.) than higher frequency sound waves.

But at the same time, the rumble of the engines may be more similar to other ambient noise like highway noise so may be harder to distinguish than train horns which are designed to be easy to hear.

The human ear doesn't perceive all frequencies equally. Human hearing is most attuned to hearing frequencies at about 3000 Hz which is roughly the pitch of a baby's cry or woman's voice. (Yes, there are lots of bad physicists jokes about that).

[MaxPC]

My observation is that the speed of sound is determined directly by the amount of interest in the listener. The speed of sound slows incredibly when the situation is a parent telling a child to clean up, for example.

There's times when the air is cold that I can hear the train horns miles out, almost to the point I can guess check crossing they are at. Other times, I can hear a low hum of the locomotives, but cannot hear the horns. My imagination?

Not necessarily. Mind you, your own body will affect your hearing. Whether your blood pressure is higher or lower that day impacts your ability to hear. Likewise any fluid build up from a cold or other condition will also impact. Mix that in with external factors and you will have a great deal of variability.

[JimFoxvog]

Colder air is also less dense so the same phenomenon applies.

Was this a typo? Isn't colder air denser than warmer air? Open a refrigerator door a crack and check above and below the door. Which is colder? The cooler air is sinking.

[Pelerin]

There's times when the air is cold that I can hear the train horns miles out, almost to the point I can guess check crossing they are at. Other times, I can hear a low hum of the locomotives, but cannot hear the horns. My imagination?

There’s other factors to consider too. When the air is cold there usually aren’t many leaves on trees to absorb the sound. If there’s snow on the ground, a lot of the local ambient noise will be absorbed and it will be easier to notice loud sounds from farther away.

[Ken]

Colder air is also less dense so the same phenomenon applies.

Was this a typo? Isn't colder air denser than warmer air? Open a refrigerator door a crack and check above and below the door. Which is colder? The cooler air is sinking.

Yes. I meant more dense, all things being equal. But it is complicated when you are talking about the atmosphere and not a lab. As you go up in elevation the air becomes both colder and less dense, for example.

[JimFoxvog]

But it is complicated when you are talking about the atmosphere and not a lab. As you go up in elevation the air becomes both colder and less dense, for example.

Yes, it's complicated. I live in a valley. It gets warmer, from evening until mid-morning, when one goes up in elevation out of the valley.

[steve-in-kville]

It seems like there is magical time as the air keeps getting cooler as dawn nears, the trains get louder. We live about 5 - 7 minutes from the tracks and some mornings the rumble through town wakes us up.